1. Field of the Invention
This invention relates to controlling rotary motors, and more particularly, to controlling the rotational speed of a fan.
2. Description of the Related Art
Fans are often used to evacuate warm air from enclosures in which electronic systems are contained. For example, most computer systems include at least one fan to aid in system cooling. The increased airflow provided by fans aids in eliminating waste heat that may otherwise build up and adversely affect system operation.
Control of fans in a system typically involves a fan control unit executing a fan control algorithm. A fan control algorithm may determine the method for controlling one or more fans that are configured to evacuate warm air from a system enclosure. For example, the fan control algorithm may specify that a fan's speed should be increased or decreased dependent upon a detected temperature. Such control algorithms may also involve turning off a fan if the temperature is deemed cool enough to do so.
Fans often include a tachometer output that provides a signal indicative of the current speed of the fan. The tachometer signal may be used to determine whether the fan is operating properly. Often, fans used for CPU and/or computer system cooling have a three-wire interface with wires for power, ground, and the tachometer signal. Fan drive systems often use a PWM (Pulse Width Modulated) signal generator to drive an external circuit that controls the voltage applied to the fan, which in turn controls the speed of the fan. PWM signal generators are useful because they provide a digital control for the pulse width of a signal. The fan is only powered for the duration of the pulse, and is typically without power between pulses. However, due to its inertia acquired while being powered, the fan will typically continue rotating even while being powered off, unless power to the fan is turned off long enough for the fan to come to a complete stop. The duty cycle of the pulse train currently being provided to the fan thus determines the fan's speed.
One problem that results from using PWM signals to drive fan circuits is that multiple cooling zones create the need for multiple sensors and multiple fans thereby causing the need for complex management schemes in fan operation management.
Many other problems and disadvantages of the prior art will become apparent to one skilled in the art after comparing such prior art with the present invention as described herein.